BASE INPUT ON FRF or RANDOM or TRANSIENT ANALYSIS

 how it is possible to assign a shift or an acceleration to the base of the structure for a FRF analysis? In particular, if i use SPCD on constrained DOF (SPC), i ve rigid body motion on SPCD dof. 

as example: 

 

NODE 1: FIX CONSTRAINTS on 6 DOFs (SPC)

NODE 1: ACCELERATION on DOF 1 (SPCD)

 

I ve a rigid body motion on dof 1.

 

I want a structure fix on constraints but with an applied acce on one or more dofs (that are constrined)... like a Rensponse Spectrum Analysis.

 

thank you to all..